1. Field of the Invention
This invention generally relates to all-glass multiple sheet glazing units and more particularly to an improved method for processing the tubular metal inserts used to form apertures, opening into the interior thereof.
2. Description of the Prior Art
As is known, one form of multiple sheet glazing unit generally comprises two sheets of glass in spaced face-to-face parallel relation having their peripheral edge portions fused together to form a continuous fused edge wall around the periphery of each of the units. The air space formed between the two sheets of glass is dehydrated or purged through at least one tubular metal insert which is sealed into the fused edge wall of the units during their fabrication and, following purging, the tubular metal insert is plugged to hermetically seal the air space to provide the desired insulating and condensation preventing quality of such units.
In the past, as disclosed in U.S. Pat. Nos. 3,027,607 issued on Apr. 3, 1962 and 3,557,400 issued on Jan. 26, 1971, the tubular metal inserts have been coated with a layer of glass having the same composition as the glass sheets forming the glazing units in an effort to form a satisfactory seal between the inserts and the glass sheets. Also these patents disclose that it is important in order to maintain a suitable sealing relationship that the thermal expansion characteristics of the tubular metal inserts closely approximate those of the glass sheets.
However, it has been found that when glass produced by the float process is used for the frit coating on the metallic insert, the glazed glass layer does not securely bond to the surface of the metallic insert and the glass layer contains significant amounts of gas. Accordingly, during the edge fusing operation of the glazing units significant amounts of gas evolve from the layer of glass on the metal inserts. The gas evolved during assembly of the coated inserts into the fused edges of the glass sheets causes foaming to occur between their interfaces, thereby producing an ineffective seal. It is believed, for some reason not known to applicants, that the amount of gas evolvement from the glazed inserts is due to the method of producing the glass; that is, glass which has been produced by the float process and glass which has been produced by the sheet or window process. In other words, it appears that the amount of gas evolved from the glazed glass inserts is dependent upon the glass used in glazing the inserts, its melting, refining and manufacturing history as well as its composition.
While the procedures disclosed in the above-mentioned patents have proven effective in applying a glass coating to the surface of the metal inserts in the past from frit produced from sheet glass, they have not proven satisfactory in providing a glass coating from frit produced from float glass in that such coatings will not form a hermetic seal between the interfaces of the inserts and the fused edge of the glazing unit because of the gaseous occlusions therein. It has been discovered that the fusing of a layer of glass produced by either aforementioned process, on the inserts to the glass sheet of the glazing units can be improved by the novel procedures of this invention with respect to gaseous occlusions in the glass layer, bonding of the glass layer on the metal insert, and resistance of the glass layer to deterioration during storage prior to use, i.e. their exposure to temperature and humidity conditions over a period of time.